Organotypic cultures of fetal mouse spinal cord with attached dorsal root ganglia (DRGs) and cultures of dissociated DRG neurons will be utilized to investigate the biochemical basis for the acute depressant effects and the tolerance that occurs during chronic exposure to opioids and to certain other receptor agonists. Acute exposure of cultures to opioids results in depression of sensory-evoked dorsal-horn synaptic network responses; serotonin (5HT) and alpha2-adrenergic agonists produce similar effects. Increasing intracellular cyclic AMP attenuates these depressant effects. The explants contain a naloxone-reversible, opioid agonist-inhibited adenylate cyclase (AC); 5HT, and norepinephrine also inhibit this AC. Chronic exposure of explants to morphine attenuates the opioid-depressant effects and produces "cross-tolerance" to depression by 5HT and other agonists. Chronic morphine treatment also enhances AC, attenuates opioid inhibition of AC and decreases the level of serotonin receptors. Many of the electrophysiological and biochemical effects of chronic morphine treatment are mimicked by treatment with pertussis toxin, an agent that inactivates the guanine-nucleotide regulatory protein, G., linking opioid and other receptors to AC. We propose to characterize the receptor subtypes that are linked via G proteins to AC and possibly other second messenger systems mediating these depressant effects of opioid and monoaminergic agonists. We will determine their distribution in the DRG and cord tissues in these explants and in cord and sensory ganglia obtained directly from the animal. Spinal cord and sensory ganglia obtained directly from the animal will also be used to examine the relationship between opioid and serotonin receptor systems. Studies will focus on the biochemical basis, including the participation of receptors. G proteins, and second messengers, for development of tolerance to opioids and "cross- tolerance" to other receptor-agonists due to chronic opioid treatment of the cultures. Additional experiments will concern receptor subtypes, G proteins, and opioids and other receptor agonists, observed in many of the DRG neurons in those explants. These studies should provide valuable insights into the molecular basis for opioid action, tolerance and plasticity of CNS opioid systems. Studies of cross-tolerance may suggest new strategies for intervention during opiate withdrawal and for counteracting tolerance when chronic opiate treatment is needed.